Colchicine at final concentration 10 to the minus 5th power - 10 to the minus 7th power M inhibited clot retraction in the diluted PRP samples (50,000 cells/mm3), but not in the normal PRP samples (cell count 200,000-400,000 platelets/mm3). Incubation with 2-deoxy-D-glucose (final concentration of 5 micron M/ml) or antimycin (0.8 microgram ml) alone failed to inhibit clot retraction in either the normal or the diluted PRP samples. However, a combination of both inhibitors retarded clot retraction. Colchicine which disintegrated platelet microtubules delayed the development of the constriction of the platelet cell bodies during clot retraction. Colchicine at concentrations which inhibited clot retraction also completely abolished the formation of platelet "spiny pseudopods. The average cpm (counts per minute) was identical for the whole platelet preparation and the TCA-precipitated fractions that were sampled between 0 - 5.0 minutes after incubation. Thereafter, there was a progressive increase in radioactivity in the whole platelet preparation, but no change in radioactivity was found in TCA- precipitated fractions. Between 0 - 60.0 minutes incubation intervals, the range of cpm was 900 - 5500 cpm/10 to the 9th power cells for whole platelet preparations and was 900 - 2500 cpm/10 to the 9th power cells in TCA-precipitated fractions. Thrombin at 0.01 - 1.0 U/ml had no effect in the colchicine binding to the whole platelet as well as to the TCA-precipitated fractions.